Load asset files with ending ".mjs" as modules

When script files in the `assets` folder Aare loaded and the file
ends in ".mjs", the `type="module"` attribute is added to the `<script>`
tag.

fix #1904

Author: jowlo

CHANGELOG.md

@@ -30,6 +30,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ## Changed
 - [#2652](https://github.com/plotly/dash/pull/2652) dcc.Clipboard supports htm_content and triggers a copy to clipboard when n_clicks are changed
 - [#2721](https://github.com/plotly/dash/pull/2721) Remove ansi2html, fixes [#2613](https://github.com/plotly/dash/issues/2713)
+- [#2730](https://github.com/plotly/dash/pull/2721) Load script files with `.mjs` ending as js modules
 
 ## [2.14.2] - 2023-11-27
 

dash/dash.py

@@ -855,9 +855,20 @@ def _relative_url_path(relative_package_path="", namespace=""):
                 raise Exception("Serving files from absolute_path isn't supported yet")
             elif "asset_path" in resource:
                 static_url = self.get_asset_url(resource["asset_path"])
-                # Add a cache-busting query param
-                static_url += f"?m={resource['ts']}"
-                srcs.append(static_url)
+                # Import .mjs files with type=module script tag
+                if static_url.endswith(".mjs"):
+                    srcs.append(
+                        {
+                            "src": static_url
+                            + f"?m={resource['ts']}",  # Add a cache-busting query param
+                            "type": "module",
+                        }
+                    )
+                else:
+                    srcs.append(
+                        static_url + f"?m={resource['ts']}"
+                    )  # Add a cache-busting query param
+
         return srcs
 
     def _generate_css_dist_html(self):